Linux is a Unix
There used to be a meme going around to the effect that Linux isn't Unix and you shouldn't call it one (I say 'used to be' because I haven't seen it recently). This has always irritated me; not particularly because I'm a Linux partisan, but because it's such a small minded claim for Unix people to make.
When you ask people to support this, they generally point at two reasons: Linux is not entitled to use the Unix trademark, and Linux's code (often just kernel code) is not descended from Research Unix.
I do not understand Unix people who can make the first argument with a
straight face. Leaving aside the fact that most of the iconic versions
of Unix have not actually been able to call themselves Unix™,
these people claim to believe that this narrow legality is more
important than everything else that goes into Unix, that a sticker
from some entity is more important than
creat() and PID 1 (to pick examples at random).
Or in short: Unix is more than Unix™, and always has been.
The second argument presumes that there is some magic contamination that has passed through the (kernel) code (even though no original AT&T code remains in things like the *BSDs by now) that was not transmitted through all of the things that the Bell Labs people wrote and people's experiences with Unix and so on. Having this now invisible line of descent somehow makes you special, no matter how little your kernel or overall source base resembles V7 or 4.2 BSD by now, while not having it is somehow so big a defect as to overcome all other points.
To me something is a Unix if it feels like Unix; if it follows the spirit, if it behaves pretty much like other Unixes, if I can find my way around it without problems.
Sidebar: to nitpickers on the Unix trademark
In theory the Unix™ trademark is currently attached to passing a set of compatibility tests. However, people don't point to the compatibility tests (because then Linux might qualify, along with less appetising things); they point to the trademark, which is subject to changing policies (and has changed policies in the past). Also, I believe that no current Unix passes those tests; there is an explicit grandfather clause for things descended from V7, because otherwise there pretty much would be no Unix™es.